1. Field of the Invention
The present invention relates to a drive circuit for brushless DC motors.
2. Description of the Prior Art
Since no brush is employed, DC brushless motors are advantageous in many ways over brush-type DC motors in that durability is increased, and motor thickness is decreased by an amount corresponding to the use of brushes thus making it easier to form the motor into a flat shape. Brushless DC motors are widely used in such applications as floppy disc drives, VTR, etc., where the increased durability and the thinner shape are required.
However, the brushless DC motor is greater in inertia than the brush-type DC motor and also the conventional brushless DC motor drive circuit employs switching elements such as transistors in place of the brushes with the result that the reverse rotation cannot be satisfactorily effected by simply reversing the polarity of the motor terminal voltage as in the case of the brush-type DC motor.
In this case, the switching operation by the transistors is a unidirectional switching and therefore the timing of the switching must be changed to effect the reverse rotation, thus making the control circuit very complicated. While the desired bidirectional switching can of course be accomplished with the use of specially designed transistors or FETs, this also gives rise to various problems such as the increased cost, the difficulty in using ICs, the need to use a bipolar power source, etc.
Also, where the brushless DC motor is subjected to servo control, if an external torque is applied in a direction to accelerate the motor, it is impossible to supply the current in the reverse direction with a simple construction and thus it is difficult to suppress the occurrence of irregular rotation due to the external torque in the acceleration direction.
Furthermore, recently the marketing of small electronic still picture cameras incorporating recording means consisting of a magnetic disk has been investigated and also portable cassette tape players have been placed on the market. With these devices, it is desirable to use a brushless DC motor for the purpose of increasing the durability and decreasing the size of the device. However, due to the facts that the electronic still picture camera is held by one hand and used to make a follow shot and the like, that a large acceleration is imparted to the tape player causing a disturbance torque which is much greater than in the case of the floppy disk drive, VTR, etc., and that the disturbance torque also acts in the acceleration direction, the conventional drive circuit is not suitable since it is difficult to control the irregular rotation due to the disturbance torque in the acceleration direction.
FIG. 1 is a circuit diagram of a conventional brushless DC motor drive circuit. The Figure shows by way of example the drive circuit for a 6-pole, 2-phase brushless DC motor.
The rotational angular position of a rotor ROT is detected by Hall effect elements H.sub.1 and H.sub.2 which are arranged to provide a phase difference of 90 degrees in terms of an electrical angle. Stator coils SC.sub.1 and SC.sub.2 are wound on the stator and terminals L.sub.1, L.sub.2, L.sub.3 and L.sub.4 of these coils are each connected through two of switching transistors T.sub.1 to T.sub.8 to a controlling circuit 1 and the switching of the transistors T.sub.1 to T.sub.8 is controlled in accordance with the outputs of the Hall effect elements H.sub.1 and H.sub.2 (the angular position of the rotor ROT). It is constructed so that the transistors T.sub.1 to T.sub.8 are supplied with current through a single power amplifier 2.
Now noting the four transistors T.sub.1 to T.sub.4 associated with the stator coil SC.sub.1, a potential difference of +V.sub.cc is produced between the terminals L.sub.1 and L.sub.2 when the transistors T.sub.1 and T.sub.4 are turned on and the transistors T.sub.2 and T.sub.3 are turned off and a potential difference of -V.sub.cc is produced between the terminals L.sub.1 and L.sub.2 when the transistors T.sub.2 and T.sub.3 are turned on and the transistors T.sub.1 and T.sub.4 are turned off. Also, when the four transistors T.sub.1 to T.sub.4 are all turned off, an open condition is produced between the terminals L.sub.1 and L.sub.2.
FIG. 2 shows an equivalent circuit of the drive circuit in the above-mentioned condition. As will be seen from the equivalent circuit, each of the transistors is equivalent to a switch connected in series with a diode.
However, if the working voltage of the motor is low, the diode is rendered in operable due to its dead zone and in fact there results the same condition as if the diode were not connected. Therefore, when the transistors T.sub.1 and T.sub.4 are on, for example, it is possible to effect the current flow only in a direction from the terminal L.sub.1 toward the terminal L.sub.2, thus failing to produce a reverse torque in the rotor ROT. While it is of course possible to produce the desired reverse torque through the controlling circuit, this complicates the controlling circuit. As a result, it is in fact very difficult to overcome any irregular rotation due to the disturbance torque in the acceleration direction only with the use of the ordinary servo circuit. Thus, it is conceivable to provide, for example, a separate brake unit utilizing mechanical friction and in this case the drive mechanism on the whole is also complicated due to the use of the separate unit.